target-arm: aarch64: add support for 1-src RBIT insn

this adds support for the C5.6.147 RBIT instruction.

Signed-off-by: Alexander Graf <agraf@suse.de>

[claudio: adapted to new decoder, use bswap64,
          make RBIT part standalone from the rest of the patch,
          will handle REV in a separate patch]

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>

From Alexander Graf <agraf@suse.de> aarch64 series 46/60